In the beginning they called it "stress relief". After the war in Wutai, they had both needed it, and as neither one of them were prone to talking out their feelings, this worked. It was the most logical solution.
Later, Sephiroth reasoned that it continued out of convenience. They both lived on the same floor, and as they were both Firsts, there was little chance anyone would find their arrangement improper. Attending to a biological need was understandable, and besides, with both of them being as enhanced as they were, it would be unsafe to engage in these sorts of activities with either civilians or lower ranked members of SOLDIER.
And if he sometimes heard rumors that Zack was sleeping with others, what of it? There wasn't anything really between them, not really. And besides, he'd heard enough rumors about himself to know that there was generally little substance to that sort of thing. How many times had he heard rumors about himself sleeping with Genesis, or Angeal, or heavens forbid, that one about himself and Hojo?
So this works. Normally Zack comes to his apartment, they sleep together, Zack leaves. Sometimes Sephiroth thinks to ask Zack to stay, just for a little bit, just for the night maybe.
When he watches Zack's back, watches him gather his clothes and go, then sometimes he doesn't know what to call it at all.
Zack makes him feel his age, which is very, very young. Sephiroth stands ramrod straight, speaks little except on military strategy, and unblinkingly looks people directly in the eye. A lot of people have taught him this speaks of maturity, and while it does the trick at work, he has frequently heard people describe it as "unnerving" when they think he's out of earshot.
It doesn't matter anyway, as long as they follow his orders. After Wutai they have so many fresh faced recruits who regard him as a hero, he could probably say anything to them and get away with it. He considers just speaking his mind during a welcome speech to the newest batch of cadets, but reconsiders immediately. He knows there are consequences to things like that, and that he is more or less company property. Still, it doesn't mean he hasn't envisioned a speech more to his liking numerous times.
Zack never holds back what he's thinking, which Sephiroth simultaneously appreciates and abhors. The first time they slept together, Zack said, "So the hair color is natural then. Who'd a thunk it?" He had only been grateful that they hadn't bothered with the lights, and that maybe Zack couldn't see the intensity of his blush. He didn't know whether he was being laughed at, or if he was meant to laugh with, but thankfully Zack had kissed him soon enough that he didn't even have to bother with a response.
The first night that Zack stays, it's more or less by accident, Sephiroth thinks. They fuck, as per usual, but Zack is unusually breathless after despite not exerting any more energy than usual. Sephiroth thinks about asking if he's feeling all right, wonders if that's a socially appropriate thing to ask your partner directly after sex, but Zack answers before he can say anything.
"Damn," he says while wheezing out a laugh. "I think I need some fresh air or something, ya know? I feel like I'm not getting enough air recently."
"I can open a window, if you'd like? Then again, I'm not sure Midgar's air is quite what you had in mind."
"Yeah, no, it can't hurt, right? We're high enough up the air's probably kind of fresh. I don't know if you appreciate it, not having done the whole cadet barracks thing, but living up this high is nice. I meant what I said earlier about them spoiling you."
Sephiroth moves to open up the window and considers telling Zack that this kind of luxury is new to him too, that he'd grown up in the labs and had only gotten this sort of freedom after he'd proven himself in the war. He'd mentioned it to Angeal and Genesis once when they'd been reminiscing about their time growing up in Banora, but found out that that's the sort of thing that ends a conversation. They exchanged looks, fumbled for words, the conversation fizzled out. Sephiroth gave a halfhearted excuse and left. He'd made note of that for future reference. Do not discuss growing up in the labs. Do not discuss how you never knew your parents. Do not discuss the inordinate amount of time you have spent wondering if there's ever been a group of people out there, past or present, who look anything like you. He's learning. Let Zack think what he will.
"I think there's something going around. Um, some kind of cold or something? You should be fine, I'm sure, I just hang out with this cadet sometimes. His immune system sucks. So, um, there's that possibility…"
"I believe sleep is the remedy for that. And soup?"
"Yeah. Ugh, I am way too tired to get up and get ready for bed. And thinking of soup from the mess hall does not make me feel any better."
"You can remain here tonight. You should be better in the morning."
Zack doesn't say anything, but relaxes into the sheets a little bit more. His breathing is still faster than it should be, but is slowing down enough that Sephiroth sees no need to worry further. He settles into bed, keeps an appropriate amount of distance between himself and Zack, and keeps perfectly still for the rest of the night.
Let no one say that Sephiroth is not considerate when sharing a bed. The same cannot be said for Zack, who spent the entirety of the night tossing and turning, sometimes sitting upright and staring straight ahead for an hour at a time. Sephiroth had been constantly aware of him throughout the entire night, and wondered if the joys of sharing a bed with someone had been grossly overstated. By the time dawn hits, there's not much point in pretending to be asleep anymore.
"I think that maybe I pulled something in my back last night." Zack says it quietly and words it like a question, as if he is hoping for some kind of confirmation. His breathing is shallow, and his teeth are gritted in a way that belies the casual nature of his words.
Sephiroth sees no point in not mincing his words. "You look awful."
"Well, ouch. You sure know how to kill a guy's self esteem. You thought I looked fine last night."
"I mean that you look pale and are in obvious pain. It might be in your best interest to consult a doctor."
"Is that an order?"
"Is that what it takes to get you to look after your own health?"
Sephiroth experiences a moment of irritation at both himself and Zack: at Zack for his current stubbornness, and at himself for not noticing Zack's worsening condition over the course of the night, having been too annoyed at the fact that all the tossing and turning lead to no sleep for either one of them. Zack looks noticeably worse now though, constantly shifting in an attempt to find a comfortable position, a light sheen of cold sweat clinging to his skin. Mako works well against injuries and airborne toxins, but generally speaking has little effect against disease. A flutter of worry flares up in Sephiroth's chest.
"...Okay, yeah. No, it should be in and out. If they asked how this happened though, I'm not owning up to anything."
"I can take you over to the medical ward as soon as you're dressed," Sephiroth says quietly. "I have a morning meeting that I'm required to attend, but I can return around lunchtime if you're still there. My apologies. I'm aware Angeal would be the ideal choice, but I'll be sure to alert him to your condition as soon as he returns from his mission. He should be no later than early morning tomorrow."
"You don't have to. You're busy."
"Ensuring the health of one of Shinra's top SOLDIERS is a priority."
Whatever the correct thing to say is, it isn't that.
Author's Note: I am probably too old to be writing this sort of thing, but I a.) still hold a soft spot for FF7 fanfiction, which is something I never want any of my loved ones to know, even if I'm dead and b.) I recently got out of the hospital for blood clots in my lungs and there's really no way to talk about that sort of thing to friends and family without generally upsetting them or making them uncomfortable. So, like a true adult, I'm using fanfiction to sort things out, all while making fictional characters fall in love (and angst a lot in the process).
I'm not super familiar with Crisis Core, but from the few scenes I have watched, it seems like Zack and Sephiroth have a somewhat contentious relationship. I'm starting with that, so no, Zack won't be a dick throughout the entire thing. I'm also writing Seph as totally awkward because I both sort of relate, and also who, except for someone who's totally socially inept, angsts about not knowing their hometown/mother to some cadet they don't even really know (yeah, I'm looking at you, Nibelheim flashback scene from the original game)? Anyway, let me know what you think. I have a vague idea where I'm going with this, but would like to know if it seems worth continuing or not.